A fuel cell essentially consists of two catalyst electrodes and a solid electrolyte membrane sandwiched between the electrodes. Hydrogen, the fuel, is ionized at one of the electrodes, and the hydrogen ions diffuse through the solid electrolyte membrane and combine with oxygen at the other electrode. When the two electrodes are connected through an external circuit, an electric current flows and electric power is supplied to the external circuit. Here, the solid electrolyte membrane has functions to diffuse the hydrogen ions, as well as to physically isolate the fuel gas (hydrogen) and oxygen and to block the flow of electrons.